Advances in the field of semiconductor devices and other electronic components along with the considerable development of printed wiring substrates on which these electronic components are mounted have been behind the remarkable progress achieved in the field of electronic devices. Since many of these electronic devices are required to be more compact, have reduced thickness and be lightweight while also offering improved productivity, printed wiring substrates have been required to incorporate various contrivances and improvements in accommodation thereof. In particular, the mounting of electrically conductive wiring forming materials of electronic components is being required to offer greater speed and higher density as a result thereof.
Amidst such circumstances, if a material were available that enabled wiring forming materials to be mounted at even lower temperatures, since fine electronic circuits would also be able to be formed on various types of easily processed organic polymer substrates such as polyethylene terephthalate (PET) or polypropylene having heat resistance lower than polyimides in addition to polyimides already used as flexible printed wiring substrates, there are considerable expectations placed on the appearance of such a new material. Accompanying advances in inkjet printing technology, nanosized metal fine particles are currently expected to serve as such a material, and studies for that purpose are currently being conducted (see, for example, Non-Patent Document 1). However, since the minimum sintering temperature thereof has thus far been determined to be only 150° C., efforts are continuing to seek out a material capable of being sintered at even lower temperatures. In addition, the generation of waste products containing by-products formed from the large amounts of reaction solvents, surplus protective molecules and reducing agents used to synthesize these metal fine particles is also required to be improved from the viewpoints of conserving resources and ecology.
According to studies conducted thus far, Patent Document 1,for example, discloses a method for manufacturing composite metal nanoparticles comprising heat treatment of two or more types of transition metal salts and amine compounds in an inert atmosphere. It is disclosed that nanoparticles are formed having a particle diameter on the nanometer (nm) level, the nanoparticles are dispersed in a somewhat non-polar solvent such as toluene, hexane, ligroin, petroleum ether or ethyl ether, and the nanoparticles are sintered by heat treatment at 200° C. to 300° C. However, it is unclear as to whether or not these nanoparticles exhibit electrical conductivity. In addition, since the heat treatment temperature is 200° C. or higher, only a polyimide and the like having considerably high heat resistance can be used even for an organic substrate having favorable processability.
In addition, Patent Document 2 discloses that coated silver nanoparticles having a uniform particle diameter are only obtained in the case an oleylamine, which is an unsaturated long-chain aliphatic amine, as an essential constituent is reacted with silver oxalate and a saturated aliphatic amine. However, dispersibility of the resulting coated silver nanoparticles in solvent, their sintering temperature and their electrical conductivity are not adequately examined or disclosed. In actuality, further improvement of these nanoparticles is required.
The reason why the silver nanoparticles described in Patent Documents 1 and 2 form a metal film by sintering at a temperature of about 200° C. that is much lower than the melting point of silver is that since such silver nanoparticles have an extremely large specific surface area, the tendency for the surface area to become even less due to the surface tension thereof becomes extremely strong. Consequently, in the case of carrying out low-temperature sintering, it is necessary to impart a large specific surface area by making the mean particle diameter of the silver nanoparticles as small as possible. However, when silver nanoparticles are simply attempted to be manufactured that have a small mean particle diameter, the particles end up aggregating (sintered) in the manufacturing process thereof due to the surface tension of the silver nanoparticles, thereby resulting in the problem of only being able to obtain coarse particles that do not demonstrate any further low-temperature sintering.
In order to solve such problems and stably manufacture fine silver nanoparticles or enhance the storability of manufactured silver nanoparticles, in the silver nanoparticles described in Patent Document 2, as previously described, together with manufacturing silver nanoparticles by using an oleylamine, which is an unsaturated long-chain aliphatic amine, as an essential constituent and using decomposition of silver oxalate, the surfaces of the manufactured silver nanoparticles are protected with the oleylamine to manufacture silver nanoparticles having a mean particle diameter of about 20 nm. In the case of these silver nanoparticles, since direct contact by surfaces of the silver nanoparticles is prevented by the presence of oleylamine molecules on the surface thereof, unintended sintering and aggregation of the silver nanoparticles during the manufacturing process and storage are prevented.